Borislav Petkov
2021-Jun-11 14:05 UTC
[PATCH v4 2/6] x86/sev-es: Disable IRQs while GHCB is active
On Thu, Jun 10, 2021 at 11:11:37AM +0200, Joerg Roedel wrote:> From: Joerg Roedel <jroedel at suse.de> > > The #VC handler only cares about IRQs being disabled while the GHCB is > active, as it must not be interrupted by something which could cause > another #VC while it holds the GHCB (NMI is the exception for which the > backup GHCB is there). > > Make sure nothing interrupts the code path while the GHCB is active by > disabling IRQs in sev_es_get_ghcb() and restoring the previous irq state > in sev_es_put_ghcb().Why this unnecessarily complicated passing of flags back and forth? Why not simply "sandwich" them: local_irq_save() sev_es_get_ghcb() ...blablabla sev_es_put_ghcb() local_irq_restore(); in every call site? What's the difference in passing *flags in and have the get_ghcb/put_ghcb save/restore flags instead of the callers?> -static __always_inline struct ghcb *sev_es_get_ghcb(struct ghcb_state *state) > +static __always_inline struct ghcb *sev_es_get_ghcb(struct ghcb_state *state, > + unsigned long *flags) > { > struct sev_es_runtime_data *data; > struct ghcb *ghcb; > > + /* > + * Nothing shall interrupt this code path while holding the per-cpu > + * GHCB. The backup GHCB is only for NMIs interrupting this path.Hmm, so why aren't you accessing/setting data->ghcb_active and data->backup_ghcb_active safely using cmpxchg() if this path can be interrupted by an NMI? -- Regards/Gruss, Boris. https://people.kernel.org/tglx/notes-about-netiquette
Joerg Roedel
2021-Jun-11 14:20 UTC
[PATCH v4 2/6] x86/sev-es: Disable IRQs while GHCB is active
On Fri, Jun 11, 2021 at 04:05:15PM +0200, Borislav Petkov wrote:> On Thu, Jun 10, 2021 at 11:11:37AM +0200, Joerg Roedel wrote: > Why not simply "sandwich" them: > > local_irq_save() > sev_es_get_ghcb() > > ...blablabla > > sev_es_put_ghcb() > local_irq_restore(); > > in every call site?I am not a fan of this, because its easily forgotten to add local_irq_save()/local_irq_restore() calls around those. Yes, we can add irqs_disabled() assertions to the functions, but we can as well just disable/enable IRQs in them. Only the previous value of EFLAGS.IF needs to be carried from one function to the other.> Hmm, so why aren't you accessing/setting data->ghcb_active and > data->backup_ghcb_active safely using cmpxchg() if this path can be > interrupted by an NMI?Using cmpxchg is not necessary here. It is all per-cpu data, so local to the current cpu. If an NMI happens anywhere in sev_es_get_ghcb() it can still use the GHCB, because the interrupted #VC handler will not start writing to it before sev_es_get_ghcb() returned. Problems only come up when one path starts writing to the GHCB, but that happens long after it is marked active. Regards, Joerg